Electrical and optical properties of undoped n-AlGaN films with Al composition close to 40% were studied before and after implantation of various doses of 100 keV protons. In the virgin samples, the electrical properties were determined by deep donor defects with-an energy level near 0.25 eV from the conduction band edge and a concentration of similar to10(18) cm(-3). Other deep centers present had energy levels of 0.12, 0.3, and 0.45 eV The luminescence spectra were dominated by two defect bands near 2.3 and 3.6 eV. Proton implantation significantly decreased the concentration of major donors even at the lowest doses of 10(12) cm(-2). For higher doses the Fermi level became progressively deeper and the data indicated complexing of defects present in the sample with either primary radiation defects or/and hydrogen introduced by implantation. The effect of the proton implantation-on the intensity of luminescence bands was complicated but overall the intensity of defect bands was increased with implantation. (C) 2003 American Vacuum Society.